Before deciding you’re too fat, it’s important to calculate your percentage of body fat. A woman can appear overweight and have a low body-fat rate, while an average-weight woman can carry a higher percentage of body fat. According to news posted on the Mayo Health System website, more than half of Americans considered to have average body weight really have unhealthy levels of body fat.

We often use weight and body fat interchangeably, write Robert Ferguson and Krista Clarke in their book Fat That Doesn’t Come Back. “But the truth is, there is a significant distinction between what you weigh and the amount of fat on your body,” they write. “Learning more about this distinction is of great importance and will prove more valuable than the confirmation you get from a bathroom scale.”

So, how do you calculate your body fat without a personal trainer or fancy equipment?

Hugo Rivera, a noted body builder with engineering and training certifications, developed a formula for calculating your body fat that’s accuracy within 3%. Gather pencil and paper, a measuring tape and a calculator and start calculating:

Step 1: Measure in inches the distance around your wrist, waist, hips and forearm. Also –write down yourweight.

Step 2: Multiply your weight by 0.732 and multiply that number by 8.987.

Step 10: Divide the Step 9 total by your actual body weight. The sum is your percentage of body fat.

Step 11: Know Your NumbersAccording to the World Health Organization, women between 20 and 40 years old should carry a body-fat ratio of 21 to 33%. Women who are between 41 and 60 years old should try to maintain a body-fat ratio of 23 to 35%. Serious health issues can occur for women who are significantly below or above healthy body-fat percentages.

If you calculate an extremely high or extremely low body fat, seek professional medical advice. Type 2 diabetes, heart disease, high blood pressure and stroke can all result from excess body fat. A brief period of higher body fat because of illness or other inability to reduce fat should not be a worry. But an extended period of excess fat causes your body to work harder and less efficiently, leaving you more prone to serious disease.